Geoscience Reference
In-Depth Information
Figure 5 presents horizontal cross-sections of wind and reflectivity fields over France at 2
km MSL and different stages of the storm evolution. At 3 UTC (Fig. 5a-b), a large part of the
country was already affected by weak to moderate precipitation. The most intense rainfall
occurred approximately 300-400 km in advance of the cyclone center along a SW-to-NE
oriented rainband that marks the location of the cyclone associated cold front. Another area
of intense precipitation associated with the cloud head (Browning 1999) could also be
identified farther west. Both regions of intense precipitation were separated by an area of
weaker precipitation associated with the dry intrusion identified in Fig.5a. Relatively
intense winds in the range of 25- 30 m.s -1 could already be observed in the southeastern
France (east of the cold front) and within the cloud head.
According to radar data, Klaus made landfall ~ 3 hours later near the city of Nantes (Fig.
5c-d). The closed circulation associated with the cyclone center can be clearly identified in
Fig 5c. The diameter of the vortex deduced from radar observations was about 350 km.
Severe winds reaching up to 50 m.s -1 have already penetrated deeply over land as seen by
the patch of very intense winds located ~400 km of the cyclone center from each side of
the dry slot. The location of this area of particularly strong winds is in good agreement
with surface observations (Fig. 4). One can also notice the presence of several thin
rainbands located to the west of the wind maximum, within the cloud head. This banded
structure is consistent with that described by Browning (2004) and suggests the existence
of multiple mesoscale slantwise circulations, which may have played an active role in
strengthening the damaging winds (the investigation of processes at play during this
event is outside the scope of this study). After landfall the patch of strong winds stretched
out along a NW-SE axis more or less parallel to the orientation of the Pyrenees mountain
chain and slowly progressed southeastward towards the Mediterranean coast. Strong
winds remained active during the entire period. A wind maximum of up to 55 m.s -1 could
be observed west of the Toulouse radar at 09UTC ( x ~ 400 km, y ~ 200 km) in good
agreement with surface observations (Fig. 4).
As there is currently no way to collect wind measurements at the space-time resolution
achieved by ground-based Doppler radars, the validation of such operational wind data is
extremely difficult. In order to evaluate these results, we propose to compare retrieved radar
winds with those analyzed by the French operational numerical weather prediction system
ALADIN (Aire Limitée Adaptation Dynamique Développement International; Radnóti et al.
1995). ALADIN is a limited area regional model that covers France and part of Western
Europe at the horizontal resolution of 10 km. The comparison between radar-derived and
analyzed horizontal winds at 1.5 km MSL at 6 UTC is shown in Fig. 7. Overall, the location
of the cyclone center, the dimension of the vortex, and the intensity and direction of the
winds at mid-level appear quite similar in both analyses. Some discrepancies can yet be seen
along frontal boundaries (the wind shift associated with the cold front is for instance slightly
more marked in the radar analysis) and to the North of the Massif Central Mountains, about
500 km east of the cyclone center. In the latter area the model produces a pronounced zonal
wind component that is apparently not resolved in the radar analysis. This pronounced
southerly component was nevertheless missing in all 15' analyses produced between 4 and 8
UTC. Although no strong conclusions can be inferred from this observation, this temporal
consistency of the wind field may plead for an error in the analysis rather than in the
retrieved winds.
Search WWH ::




Custom Search